Electric Motors

Hi gents, been researching single phase versus 3 phase electric motors for my lathe. What are the advantages and disadvantages of both systems? As far as my understanding, 3 phase is better especially with an inverter. Down side is the cost although its worth investing. I have a single phase motor which is ok but tends to get hot to touch and sometimes stops working. Its still manageable but I guess it won't be long until it stops working. I'm no expert in motors and trying to educate myself but would be good to hear from all of you. At the moment budget for a whole 3 phase systems with an inverter is not possible. I have managed to get a good second hand 1 hp single phase motor. Can this be used with an inverter? I read that a single phase is not compatible with an inverter but looking at Newton Tesla website they have a single to 3 phase inverters. Can this be used with a single phase motor? There is nothing in the info on the website to tell me this clearly or I'm being ignorant.

I'm going to limit my answer to induction motors (there are also lots of DC/Universal brushed and brushless motors out there).

To start an induction motor more than one phase is needed. This is either supplied from using a three phase supply or a phasing capacitor on a second coil driven by a single phase supply. (Just realised you can have shaded pole single phase motors but they typically only work in fractional horsepowers.)

Because the induction motor only needs more than one phase to start the starting capacitor and coil are sometimes disconnected with a centrifugal switch. This disconnection allows the starting capacitor and coil to be significantly derated.

The advantage of a three phase induction motor is the absence of failure prone capacitors and centrifugal switches and the ability to vary the frequency of a VFD to vary the speed. The disadvantage of a three phase induction motor is the need for a three phase power supply that is not found in the majority of houses or the purchase of a VFD.

Running a single phase induction motor on an inverter? Obviously if the inverter is running at 50Hz and can supply the starting current (maybe double the full load current) then, yes. Using the inverter to change the frequency would be an interesting experiment. Lowering the frequency would increase the capacitor's impedance and reduce the current in the phasing coil meaning the motor would be relying increasingly on a single coil. Conversely, increasing the frequency would give more current through the phasing capacitor and it's coil. It would also reduce the phase shift between the main and phasing coils, the result of this reduced phase shift I cannot confidently predict.

And before anyone flames me I have no idea whether any particular inverter will run with just two of the outputs connected to a single phase motor with the third output dangling free.

Modern induction motors can handle very high temperatures before the coil insulation fails, see **LINK** Does your motor plate give an insulation rating?

Am I right in assuming the motor is for a Myford? If so, check the motor you have and see if it is the "open" type where ventilation slots open directly into the inside area of the motor. This allows many years' build-up of swarf between the armature and windings, which will make it run hot. Solution is to strip and clean the motor. Might as well fit new bearings while it is apart.

If you have not enough money for the full three-phase inverter system, and not enough knowledge and experience to know if a hybrid system can be improvised successfully, you might be best to stick with the single phase. It is simple and effective and has worked for the past century or so on home hobby lathes.

Induction motors, if from a good manufacturer, are inherently long lived - giving many thousands of hours operation as long as routine maintenance is carried out. They normally only fail because of the incorrect operation by its users.

Bearing failure or over-loading are the chief culprits, unless operating in decidedly damp conditions (and that includes starting, without drying out, after long term storage). If your motor is stopping by disconnection of an overload trip, you need to find why - or it will eventually fail (as it seems that it is not the overload trip that is set too low on this occasion).

Running the motor completely off load for a period may (but not always) give you a clue as to whether it is an internal motor problem, or the external mechanical load that is the problem.

Single phase motors without any protective overloads are far more likely to be destroyed than those fitted with proper starters with appropriate safety devices included.

Any decent 3 phase inverter will likely trip if the phase currents are not in reasonable balance, I would think. Three phase motors are not that difficult to find, but the extra cost of a VFD needs careful consideration - as there are other benefits, for motor protection and speed control, built into them. There are non-variable single to three phase converters out there too.

Two of my machines have well documented vibration reports, when driven by single phase motors, which makes me lean toward 3 phase motors wherever/whenever possible and appropriate.

My advice would be to sort out the problem of the current motor. Not just fit a larger powered motor (if that is the case). Otherwise you may finish up with two blown motors or a damaged machine.

[ ... ] At the moment budget for a whole 3 phase systems with an inverter is not possible. I have managed to get a good second hand 1 hp single phase motor. Can this be used with an inverter? I read that a single phase is not compatible with an inverter but looking at Newton Tesla website they have a single to 3 phase inverters. Can this be used with a single phase motor? There is nothing in the info on the website to tell me this clearly or I'm being ignorant.

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Raph,

I would just like to comment on that note ^^^ before it gets lost in the noise of discussion.

The sloppy use [by suppliers] of the term 'inverter' probably gives rise to much of the confusion in this field:

The systems that are commonly supplied have fixed frequency single phase input and variable frequency three phase output ... and some of the magic in that box is achieved by inverters.

BUT describing the whole system as 'an inverter' is far from helpful: I think it probably dates back to the time when they first used 'solid state' electronics to do a job that was previously done with rotating machines !!

"I have a single phase motor which is ok but tends to get hot to touch"

My now departed WM250 suffered the same complaint. I limited this to some extent by lashing up a large computer fan on the end of the motor. (Only JUST enough room between motor and splash guard to do this!)

My WARCO bandsaw gets even hotter. After 20 - 30 minutes work (even with a new blade) the motor gets so hot I can smell the varnish burning on the windings meaning that in order to avoid flames the work has to be done in shifts while the motor cools down. Maybe the fan idea might help here as well but I am doubtful ... perhaps a round TUIT sometime

Gents, I think you should establish what sort of lathe Raph has and what he is using it for, before you start advising him on the motor.

I think there is only one post that has ignored the fact that we do not know the specific details of his machine?

As far as what he might be using it for is his own business, should he not wish to offer that information. The emphasis has rightly been on the differences between common 230V single phase domestic supply and add ons, for driving 3 phase motors, plus the potential advantages of the typical 3 phase motors in that range.

Perhaps your post should have been directed towards the OP, not the responders?

Motor temperature can be an issue with our relatively small machines, I mean in comparison to larger industrial machines, manufacturers obviously fit motors that will cope, with very little spare capacity, as a matter of economics, it eats into profits or makes their product less competitive price wise if you fit larger capacity motors. Recently I was using my Champion V20 mill for a particularly long and arduous drilling operation, the motor, which is enclosed in a fibreglass shroud, warmed up very quickly and I was fearful of overloading it heatwise so I removed the shroud and positioned a large box fan to provide a stream of cold air, it worked fine and I completed the work without any problems with the motor. I am now looking to modify the motor shroud and incorporate a small cooling fan that will run all the time the machine is switched on, it will at least give me peace of mind. I am sure that the majority of motor failures on “model engineering” size machines are due in some way to heat overloads because the operator is using the machine at the limit the motor is rated for.

INT (intermittent) intermittent use at the rated output - requires a cool down period after half an hour or so like a welder does.

My Rockwell Delta 10" behaved like your Myford, run for a while then cut out. I found that despite looking very clean there was a fair bit of plastic swarf and black dust in the motor. A good clean out has fixed the issue. That said it still runs at around 50 degC and according to the insulation letter code it's OK to about 65degC. Check the colour of the windings at the same time, any discolouration is likely to be a faulty coil.

This all assumes that nothing is over tight or running dry in the lay shaft and the motor is an internally cooled one.

It's best not to stray far from the recommended motor power for any lathe, they are not engineered for double the HP.

The trouble with VFD's is for lower speeds the motor runs slower and so tends to heat more plus cutting power goes down. Whereas using gears or belts for speeds the motor always runs at the design speed and you get more cutting power.

Here's a simplified review of electric motor types, in order of suitability. Feel free to point out mistakes!

Universal AC/DC Motor. Very common in domestic appliances. Difficult to control efficiently, poor torque, and performance varies wildly across the operating range. Fine in an electric drill or similar, best avoided on a lathe except perhaps a small one.

Single Phase AC Motors. Main advantage, they plug straight in to an ordinary domestic mains socket. Inside special arrangements are needed to start the motor (like capacitors, start/run windings, and a centrifugal switch) and these reduce reliability. Bumpy delivery of power causing vibration. Performance and efficiency satisfactory rather than good. Not popular over, say, 2000W.

Brushed DC Motor: reliable except the brushes wear out. Easy to control speed. Good torque and power characteristics. Smooth operation. Popular up to about 1100W; above that you move into a special power supply territory. Good with electronic speed control.

Three Phase Motors. Simple, highly reliable, low vibration. Good power and torque characteristics. Disadvantages, require a 3-phase supply (expensive to install), and motor speed is locked to mains frequency. However, wiring the motor via a VFD allows the motor to run from single phase, makes speed control easy, and otherwise enhances the motor. This is an excellent combination.

Brushless DC Motors. These can be excellent provided you choose the right one. (Not all variants are suitable for powering machine tools.) The name is misleading because they're more closely related to three phase types rather than a brushed DC motor. In effect they are multi-phase motors, an improvement on 3-phase, plus other advantages. Downside is they are more expensive to make than a three-phase, and they require an electronic driver. Been around for about 40 years, but take-up has been slow until recently. I think the main reason is that the necessary electronics were originally expensive and unreliable - not so today. Brushless motors are supplanting 3-phase for many applications in industry because they offer better performance and energy savings.

If I were upgrading an older lathe today, I would go for Option 4, and fit a 3-phase motor plus VFD. It's easy to do, well understood, affordable, and gives excellent results.

Buying a new lathe, I 'd look very seriously at Brushless. I'd think twice about doing an amateur brushless conversion on an older machine because the technology is outside my comfort zone.

If a motor gets hot when working hard the best thing to do is let it run with no load, just for a minute or two. Switching off when hot is not good for it. This applies to any motor, but especially to small motors such as hobby lathes, hand drills etc.

Hi everyone I'm a newbie on here I have a question on motors. Having recently brought a Ajax Halifax aj8 horizontal milling machine with an old single phase motor on it I was wondering are the bearings are oiled or greased? Their looks like 2 spring loaded points to be oiled/greased front and back? The motor is old and has a slight resistance to being rotated freely but seems to run okay? George

The safe operating temperature of a motor is defined by how well the winding insulation stands up to high temperatures. There is a letter classification defining the maximum safe internal temperature rise relative to a nominal 40°C ambient temperature (phew!). Summarising and simplifying, for modern motors this ranges from 60°C for A class to 125°C for H class. Naturally there isa safety margin before the magic smoke escapes. Obviously the outside case of a happy motor is going to be cooler than the depths of the windings but even an A class motor could be too hot to touch.

In America Underwriters Laboratories sets maximum acceptable surface temperatures for a "metal surface subject to casual contact" of 70°C (158°F) after 30 minutes of operation in a 25°C (77°F) room. Not things you'd want to touch for long!

Its probably safe to assume that any older, lower power, exposed motor on our sizes of machine would be fairly cool running and nominally A class with relatively limited temperature rise tolerance. Even so too hot to touch safely whilst running normally is plausible. I've encountered a couple of examples. One was a permanent capacitor, synthetic second phase device intended for direct coupling to a fan which would get spectacularly hot if run unloaded for more than a minute or two. A totally unsuitable motor for the pillar drill it had been bodged onto.

To Dave's round up I would add that most brushless and VFD controllers are sophisticated and 'model' how a motor heats up when run slowly, reducing their output or even stopping to help prevent overheating.

Most brushed motor controllers are far less clever and will happily let you runs nice and slow under load until the windings burn out (guess how I know this...)

Ralph, just home again.I'm not going to give a lot info about other motors, just 3 phase Yes the motor will run via an inverter provided the windings can be configured in Delta, this usually can be done in terminal box in a few minutes.Several of my friends have bought Huanyang inverters from Ebay for about £110, some came from Leicester others from Germany. All are happy with what they bought. Three phase motor are simpler than single, less moving parts consequentially are more reliable. Go for it, and if you buy a Huanyang I have simplified programming list which you can have a copy of. Making a control box is easy as you are playing with 12 volts, the switches are cheap and so is the control pot all for about a tenner and a bit of time. I'm told there is a good DVD thing on YouTube showing programming a Huanyang inverter. The output can be easily turned (power wise) within the program. If you would like a copy of the program, send me PM with your email address. John

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